This laboratory is devoted to the study of the epithelial cell regulation of differentiation by vitamins and hormones. The cell line used is a nontumorigenic mouse testicular line of Sertoli cells, TM4, which, when exposed to retinoic acid, produces the serum protease, plasminogen activator, one of the many proteins these cells may elaborate in their support of sperm development. An\important feature of these cells is that they grow in serum-free medium. This allows the study of the mechanism of action of a single specific inducer as well as the interrelationships of a range of hormones and growth factors affecting gene regulation. The main experimental approach will be the isolation and the characterization of mutants resistant to the induction of differentiation by retinoic acid. The point at which the retinoic acid-activated pathway is interrupted will be investigated by analyzing the production of plasminogen activator mRNA and by identifying protein and phosphoprotein regulatory intermediates. The impact of this work will focus on cancer in two different ways: (1)\retinoic acid has been shown to slow cancerous processes, inhibiting neoplasia and even tumor cell proliferation in certain situations. This process has been hypothetically linked to the differentiation-inducing capabilities of retinoic acid, but the path by which retinoic acid induces differentiation in any cell type is still not known. (2)\Plasminogen activator is known to be involved in tissue remodeling and has been identified as a product or agent of oncogenic transformation. It is possible that by identifying one pathway by which it is produced in normal cells, a clearer perspective can be obtained of its role in neoplastic cells. Although the reproductive tract nature of the TM4 cell line is not a major focus of this work, it is evident that in providing information about differentiation in a Sertoli cell line the experiments described here may well contribute to better understanding of sperm development. (P)